Spent fuel shipping cask

ABSTRACT

A shipping cask for spent or failed nuclear fuel assemblies, having a generally cylindrical stainless steel enclosure with a thick stainless steel slab welded to one end, a thick stainless steel closure removably sealed to the other end, a sheath of lead surrounding the cylindrical wall, separate tanks for neutron absorbing liquid surrounding the lead sheath and balsa wood impact absorbers on the exterior of the cask. The cask is also provided with pressure relief and drain valves housed in cavities in the steel for protection and trunnions specially arranged to facilitate handling.

United States Patent Rollins et a1.

SPENT FUEL SHIPPING CASK lnventors: Jack D. Rollins; Ralph E. Best, both of Gaithersburg; Robert R. Riggs, Linthicum Heights; Clifford .1. Anderson, Buckeystown, all of Md.', Victor .1. Barnhart, Boston, N.Y.

Nuclear Fuel Services, Inc., Rockville, Md.

Filed: Feb. 27, 1973 Appl. No.: 336,191

Assignee:

US. Cl 250/507; 250/518 Int. Cl. GZlf 5/00 Field of Search 250/506, 507, 518; 176/72 References Cited UNITED STATES PATENTS 7/1962 Montgomery 250/507 1/1966 Bonilla et al 250/507 111 rrr wil [111 3,886,368 [451 May 27, 1975 3,414,727 12/1968 Bonilla 250/506 3,432,666 3/1969 Nash et al.....

3,466,662 9/1969 Blum 3,483,381 12/1969 Bonilla .4 250/506 Primary Examiner-Harold A. Dixon Attorney, Agent, or Firm-Bacon 8L Thomas [57] ABSTRACT 18 Claims, 11 Drawing Figures PATENTEUEMN 1915 8.888388 SHEET 1 PATENTEUMAYZY 1915 8.888888 SHEET 3 SPENT FUEL SHIPPING CASK BACKGROUND OF THE INVENTION This invention is in the field of shipping containers, particularly for spent or failed nuclear fuel assemblies.

It is customary, in the operation of nuclear reactors, to remove the fuel assemblies after their energy has been depleted down to a predetermined level. Those fuel assemblies are customarily shipped to a processor who reconcentrates the active fuel components and uses the concentrates to construct new fuel assemblies for use in such reactors. As is apparent, the so-called spent fuel assemblies are still highly radio active and great care must be taken in their packaging and shipment to avoid fire and/or other accidents and/or radiation danger to personnel. Heretofore, extremely bulky containers have been used and the protection against fire and/or personal radiation has been a difficult prob lem.

SUMMARY OF THE INVENTION The present invention contemplates a shipping cask for spent fuel assemblies constructed of stainless steel and other materials, including lead, to serve as a gamma ray shield. The cask is further surrounded by tanks for the containment of such neutron materials as a borated water, ethylene glycol mixture. The neutron shielding material is arranged in separate tanks forming a belt around the cask and provision is made for relief of excess pressure and to prevent undue surging of the liquid along with the prevention of entry of air into the tanks. Valves are provided for draining the central enclosure ancl/or the neutron shield tanks and impact shielded relief valves are provided for the central enclosure and means are further provided to detect any leakage around the gasket of the cask closure. A unique arrangement of trunnions facilitates the bandling and positioning of the cask and balsa wood impact absorbers are provided.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of the shipping cask of the present invention;

FIG. 2 is a front elevational view, as seen from the left of FIG. 1;

FIG. 3 is a rear elevational view, as seen from the right of FIG. 1',

FIG. 4 is an enlarged fragemntary horizontal sectional view taken on the line 4-4 of FIG. 2;

FIG. 5 is a transverse sectional view, taken on the line 55 of FIG. 4',

FIG. 6 is a transverse sectional view, taken on the line 66 of FIG. 4;

FIG. 7 is a transverse sectional view, taken on the line 77 of FIG. 4',

FIG. 8 is an enlarged fragmentary vertical sectional view, taken on the line 8-8 of FIG. 6',

FIG. 9 is an enlarged fragmentary vertical sectional view, taken on the line 9-9 of FIG. 6',

FIG. 10 is a fragmentary vertical sectional view of the cask cavity, showing a modified arrangement for receiving a plurality of fuel containers; and

FIG. I1 is a sectional view of a special container for failed field rods.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring first to FIGS. l, 2 and 3. the shipping cask generally designated 2 is provided with a central enclosure to be further described in greater detail and an end closure structure 4 which will also be described in greater detail. The removable closure structure 4 is at what will be termed the top of the cask and, as can be seen from FIG. I, a pair of diametrically opposed trunnions 6 are provided. The trunnions may be engaged by a suitable yoke structure and lifting mechanism to lift the cask to or from a conveying vehicle or the like. Near its opposite end the cask is provided with a second pair of trunnions 8 extending outwardly from opposite sides thereof substantially parallel to the trunnions 6. However, as can be seen, the trunnions 8 are laterally offset from the axial center line of the cask 2. They are thus not diametrically opposed to each other as are the trunnions 6. When the trunnions 6 are engaged by a lifting mechanism and the cask is lifted from any supporting structure, the cask will hang vertically below the lifting structure. The cask may then be swung or transported to a vehicle or the like having thereon a cradle adapted to engage the trunnions 8. Thereafter, as the cask is lowered onto the cradle, its weight rests on opposite trunnions 8 and further lowering of the lifting mechanism will cause the cask to tilt toward a horizontal position in which the trunnions 6 are received by a suitable cradle means to hold the cask in a horizontal position with its four trunnions then resting on a suitable cradle. It is contemplated that such cradles be provided on any desired transport vehicle and that further means be provided to secure the cask in position on such cradle.

Referring now to FIG. 4, the cask comprises an inner generally cylindrical tube 10 of stainless steel and a further generally cylindrical tube 12 also of stainless steel radially spaced outwardly from but concentric to the tube 10. A bottom closure 14 of a thick stainless steel slab is provided with a flange l6 coextensive with the inner tubular member 10 and welded thereto. The outer tubular member 12 extends into a rabbet 18 in the slab 14 to which it is likewise welded. Thus, the slab 14 constitutes a closed end for the cask enclosure. As shown, the outer tubular member 12 is stepped" in wardly at 20 at about the region of the upper end of any nuclear fuel assembly therein. The upper ends of the tubular members 10 and 12 are respectively welded to a stainless steel flange structure 22 to thus completely close the space between the tubular members. The annular flange 22 defines the upper open end of the cask.

The space between the tubular members 10 and I2 is occupied by an annular body 24 of lead substantially completely filling the same to serve, with the stainless steel tubes, as a gamma ray shield. The thick slab 14 of stainless steel defining the bottom of the cask serves to shield the ends thereof against gamma ray radiation. The body 24 of lead substantially but not completely fills the space between the mentioned tubular members. Where the tube 12 is stepped in, at 20, the lead is thereby thinner where less shielding is required and this also reduces the weight of the cask. An annular void 26 is provided to permit thermal expansion of the lead without imposing undue stresses on its enclosing structure. The lead 24 is bonded to the shells l0 and I2 and if desired axially extending copper fins II and/or 13 may be secured to the members 10 and I2 and extend into the lead 24 to form a good thermal connection.

The bottom of the cask is defined by a stainless steel shell structure 28 spaced outwardly from the slab 14. The slab 14 is preferably formed as shown and reinforcing gusset plates 30 are welded thereto to provide rigidity for the shell 28. Within the shell 28 and opposed to the end surface of the slab 14 is a body of balsa Wood 32. PK]. 7 shows the structure in this region in greater detail. Between the balsa wood 32 and the slab I4 is a layer of asbestos 34 to serve as thermal protection for the balsa wood. The balsa wood 32 is arranged with its end grain running axially of the cask and it serves as an impact limiter of shock absorber to prevent damage to the cask. without adding appreciable weight thereto. and/or contents if the cask should be dropped while being lifted. As is apparent from FIG. 4. the trunnions 8 are welded to the shell structure 28. The shell 28 holds the wood in position and further serves as a pedestal for supporting the cask upright on a supporting surface.

The slab 14 is further provided with passageways 36 therein (see also FIGS. 3 and 7) communicating with the interior of the enclosure within inner steel tubular member 10. Those passageways communicate with radial cavities 38 in the slab l4 and aligned opening in shell structure 28 and in which suitable valves 40 are positioned and by means of which the enclosure may be drained of coolant water when necessary or desirable. The cavities 38 are closed and sealed against leakage by suitable closures 42.

At its upper end the cask is provided with a stainless steel annular shell 44 suitably welded to the annular flange 22 and the outer tubular members 12. The trun nions 6 are welded to the outer periphery of the shell structure 44. The interior of the shell 44 contains a plu rality of sectors of balsa wood 46, seen also in FIG. 5. each arranged with its grain extending generally radially of the cask and generally perpendicular to its outer surface to serve as lightweight impact absorbent means in the event the cask is dropped while tilted and to protect the cask against damage due to any lateral blows in the region of the top thereof. As clearly seen in FIG. 5, the balsa wood sectors 46 are separated by reinforcing plates or gussets 48 extending radially within the annular shell structure 44.

A thick stainless steel slab closure member 50 is configured to extend within the opening in flange 22 and is provided with an outer flange 52 overlying the outer surface of the flange 22. Preferably. the mating surfaces of the closure 50 and flange 22 are accurately machined to provide a proper seal. The closure 50 is secured to the flange 22 by means of suitable bolts 54. lnwardly of the bolts 54 is provided a pair of radially spaced grooves in which O-rings 56 are positioned to ensure proper settling of the enclosure. However. as a further safety feature. a leak detector valve is provided. The flange 22 is provided with a well or cavity 58 therein (see FIG. having a valve 60. to be further described later. communicating with a passageway 62 and a further passageway 64 exten ing through the outer face of the flange 22 into the region between the 0- rings 56. Thus. if there is any leakage past the inner O- ring. such leakage can be detected by manipulating the valve 60 to take a sample from the space between the O-rings. The valve 60 further serves as a safety valve. as will be further described later.

After the cask is loaded with its spent fuel assembly or assemblies and the closure 50 is bolted in position.

a further sheel structure 66 is secured to the upper end of the cask by bolting the same to the closure 40, as shown at 68. The shell 66 is provided with a body of balsa wood 70 therein having its grain extending perpendicularly to the outer surface of the closure 50, that is. parallel to the axis of the cask. As is obvious. the balsa wood 70 serves to absorb impacts occurring at the upper end of the cask. Access to the bolts 68 is provided by passageways 72 in the shell 66 and balsa wood 70, which are preferably thereafter sealed by suitable closure means 74 to prevent leakage of radio active materials therefrom. Both the balsa wood 46 in the shell 44 and the balsa wood 70 in shell 66 are thermally protected by layers of asbestos 76 and 78.

The space between shells 28 and 44 is provided with a further generally cylindrical housing 80 (see also FIGS. 6 and 8) divided into at least 4 axially extending compartments or tanks 82 by means of radially extending plates 84 welded to the shell 80 and tubular member 12. Thus, the shell 80 and plates 84 provide at least four circumferentially adjacent chambers or tanks 82 for the containment of neutron absorbing liquid materials. The neutron absorbing liquid is preferably a mixture of potassium tctraborate. water and ethylene glycol. ()f the total mixture. about 70 percent is ethylene glycol and about 0.6 percent is boron. Each tank is pro vided with an angled plate 86 welded therein and pro vided with openings 88 therethrough. The plates 86 provide strength and rigidity to plates 84 in the event pressure or material is lost from any of the tanks 82. At the upper end of the shell 80 a somewhat larger diameter shell is provided. as shown at 90, and a circumferentially extending radial plate 92 defines a partition separating the main tank portions 82 from surge tank portions 93 for accommodating thermal expansion of the liquid in tanks 82. Communication therebetwecn is provided by a relatively small opening 94 in the tank 92 and suitable cup structures 96, 98 (see also FIG. 8) ensure that liquid will at all times, during normal use. cover the opening 94, to prevent entry of air from the surge tanks 92 into the main tank portions 82. The arrangement is such that the opening 94 is covered with liquid for all deviations of the cask from a horizontal position up to about 45. The cups 96 and 98 are so shaped and positioned that the openings 94 will at all times be sealed against passage of air from 92 to 82. The surge tanks 93 are provided with internal baffle plates 95.

The tanks 82 terminate. at their lower ends, in a generally conical partition 99 leaving a weight reducing void at 101 within shell 80. Little neutron shielding is necessary at the lowermost end of the cask.

About midway of their length the tanks 82-92 are provided with pressure relief valves 100. set in cavities 103 for protection against external impacts. Positioning the relief valves substantially midway of the cask assures that upon rupture due to fire accidents or other causes. only half olthe neutron shielding liquid will be lost.

FIG. 9 illustrates a further safety valve and drain and fill arrangement for the tanks 82 and comprises a tubular member 102 communicating with the tank 82 and extending through surge tank 93 to a fitting 104 having a passageway 106 communicating with the exterior of the cask. Numeral 108 indicates a removable shear disc adapted to rupture upon excess pressure and which may be removed for filling and/or draining the tanks 82.

Referring to FIG. 5, there is shown. in a cavity 120 in flange 22, a pressure relief device 122 communicating with the interior of the cask enclosure in space 123 between flange 22 and closure 50. Relief of cask pressure should not normally be required under ordinary circumstances. However, in the unlikely event that pressure relief should be required. such a relief device is provided. The relief device comprises a rupture disc device 125 and a safety relief valve 127 as a back-up in series therewith. These items are separately available on the market and need not be described in further detail.

In a further cavity 124 in the flange 22, there is provided a cask vent valve 126. The relief device 122 and the vent valve 126 are rendered accessible through the illustrated openings in shell 44 and balsa wood 46 and each is closed with a sealed closure 128, as in the access opening for the gasket leak detecting valve 60; the closures 128 may be identical to closures 42 for the drain valves at the bottom of the cask.

The vent valve 126, gasket valve 60 and drain valves 40 may all be essentially of the same construction and are presently available on the market. They need not be described in detail. However, each comprises a stainless steel ball seating on a Teflon seat. The valves have a secondary metal seal downstream from the Teflon that prevents leakage in the event the valves are overheated and the Teflon sublimates. In that case, the ball moves downstream and seals against a metal lip in the valve body. The openings leading to these valves provide access whereby the valves may be manually manipulated when necessary or desirable.

It is to be noted that the valves 40, 60, 100, 122 and 126 are all housed in cavities in adjacent cask struc ture. This arrangement protects the valves against damage or destruction due to any external impacts on the cask and helps to protect them against damage if the cask should be exposed to an external fire.

Each of the valves 40, 60 and 126, of the type already described, is provided with an elongated frangible or breakaway tubular outlet 110. Thus any accidents or blows severe enough to crush shells 28 or 44 will fracture tubes 110 without damaging or opening the valves to which they are attached.

A quick disconnect valve 112 is mounted at the outer end of each tubular outlet 110. Such valves are known and each houses a normally closed valve and means by which a suitable conduit may be quickly connected thereto. Upon connection of such conduit the valve is caused to open. The valves 40, 60 and 126, as purchased, are provided with means (not shown) whereby they may then be opened to provide communication between the conduit thus connected and the space controlled by the valve.

Each of the closures 42 and 128, in addition to being sealed, is provided with suitable thermal insulation to protect its associated valve from external fires.

As shown in FIG. 4 there is a solder seal ring 114 between shells 44 and 66. During the time spent or failed fuel assemblies are being loaded into the cask, the end surfaces of the shell 44 and flange 22 are exposed to contamination by radioactive materials (the remainder of the cask is normally protected by a suitable shroud) and the solder seal thus contains and shields such contamination during transport of the cask.

The cask is further provided with openings filled with solder plugs 116, in the outer shell to serve as blow-out plugs and/or vent holes.

From the foregoing description it is apparent that applicants have provided a shipping cask for radioactive materials capable of complete containment (zero release) of the material even in the event of any probable accident while maintaining the device within legal weight limits.

Referring now to FIG. 6, numeral schematically illustrates a PWR intact fuel assembly in the cask which may be a fuel assembly as withdrawn from a reactor. Suitable positioning brackets (not shown) are provided on the inner tubular member 10 and releasably position shield and support plates 132 arranged to define a generally square compartment for a fuel assembly 130 within the cask enclosure and to provide additional radiation shielding. Holes 133 (FIG. 4) are provided in the end portions of plates 132 to facilitate circulation of coolant in the cask enclosure.

FIG. 10 illustrates an alternative arrangement wherein a pair of intact BWR fuel assemblies 134 are positioned within the cask enclosure by support and shield plates 136 held within the enclosure in a manner similar to that described with reference to FIG. 6, and also provided with openings corresponding to the holes 133.

In some instances one or more fuel rods of the assembly to be shipped has ruptured, releasing its contained nuclear fuel. Such assemblies are handled by means of a special container as shown in FIG. 11 wherein 140 is a fuel assembly having at least one ruptured fuel rod therein. The assembly is housed in a container having side walls 142 serving as gamma ray shielding. The walls 142 are secured to end frames or plates (not shown) and are spaced apart at the corners of the container. The corner openings are closed by stainless steel mesh 144 or other suitable foraminous material. The mesh serves to retain loose fuel particles while permitting free circulation of coolant therethrough. In use, the container of FIG. 11 may occupy the spaced shown in FIG. 6 as occupied by the assembly 130 and plates 132.

As indicated in FIG. 4 at 135, a pedestal or spacer of suitable length may be provided for positioning fuel assemblies in the cask enclosure.

While a single specific embodiment of the invention has been shown and described, it is to be understood that the same is merely exemplary of the invention and other forms may be resorted to within the scope of the appended claims.

We claim:

1. A shipping cask for spent nuclear fuel assemblies, comprising:

means defining a generally cylindrical enclosure having a closed end and an open end;

gamma ray shielding material surrounding said enclosure;

neutron shielding means extending circumferentially around said gamma ray shielding material comprising means defining a plurality of separate longitudinally extending compartments for bodies of liquid material capable of absorbing neutrons;

pressure relief means for each of said compartments,

and

each of said compartments comprising a main portion and a surge tank portion separated therefrom by a partition, a small passageway through said partition, and means at said passageway for holding a body of said liquid in closing relation to said passageway throughout a substantial range of tilting of said cask to thereby prevent air from flowing from said surge tank portion into said main tank portion,

2. A shipping cask as defined in claim 1 and includa first pair of radially outwardly extending diametrically opposed and axially aligned trunnions adjacent said open top; and

a second pair of opposed and axially aligned trunnions extending outwardly from adjacent said closed bottom and from opposite sides of said cask with their aligned axes parallel to the axes of the trunnions of said first pair, the trunnions of said second pair being laterally offset from the longitudinal axis of said cask whereby when said second pair of trunnions is engaged with a supporting cradle, with said cask upright, said cask will be unbalanced and tend to tilt toward a horizontal position.

3. A cask as defined in claim 1 wherein each of said compartments is provided with a solder filled opening in its outer wall, serving as a blow-out plug.

4. A cask as defined in claim 1 wherein said bodies of liquid material comprise potassium tetraborate, water and ethylene glycol.

S. A cask as defined in claim 4 wherein said ethylene glycol comprises about 70% of said body of liquid material and about 0.6% of said body of liquid material is boron.

6. A cask as defined in claim 1 wherein said compartments extend axially of said cask less than the full length of said enclosure, having their ends inwardly of the ends of said enclosure.

A shipping cask for spent nuclear fuel assemblies,

comprising:

means defining a generally cylindrical enclosure having a closed end and an open end;

gamma ray shielding material surrounding said enclo sure;

neutron shielding means extending circumferentially around said gamma ray shielding material comprising means defining a plurality of separate longitudinally extending compartments for bodies of liquid material capable of absorbing neutrons;

pressure relief means for each of said compartments;

said gamma ray shielding comprising radially spaced generally concentric tubes of stainless steel;

an annular body of lead substantially but not completely filling the space between said tubes;

a body of stainless steel defining said closed end and being welded to the adjacent ends of said tubes;

an annular flange of stainless steel welded to the other ends of said tubes and defining said open end: and

a stainless steel closure secured to said flange and being sealed thereto.

8. A cask as defined in claim 7 wherein said closure is sealed to said flange by a pair of spaced O-ring gas kets. and a gasket leak check valve carried by said flange and communicating with the space between said O-rings.

9. A cask as defined in claim 7 including metal vanes secured to at least one of said tubes and extending into said body of lead to enhance heat transfer therebe tween,

10. A shipping cask for spent nuclear fuel assemblies, comprising:

means defining a generally cylindrical enclosure having an open top and closed bottom and comprising thick metal members, of gamma ray shielding material, defining said closed bottom and said open top;

a radial cavity extending inwardly from the outer surface of said cask into at least one of said members;

a passageway extending inwardly from said cavity to a space within said cask; and

valve means for closing said passageway, said valve means being entirely within said cavity within said gamma ray shielding material and thereby protected from damage by external impacts.

1]. A cask as defined in claim 10 wherein said valve comprises a rupture disc device adjacent said passageway and a pressure relief valve in series therewith.

12. A cask as defined in claim 10 including an annular structure around said one member;

an opening through said structure and communicat ing with said cavity;

said valve having an outlet port and a frangible tube extending outwardly from said outlet port into said opening; and

a normally closed, selectively openable valve at the outer end of said tube adjacent the outer end of said opening 13. A cask as defined in claim 12 including a removable closure sealingly closing the outer end of said opening and including thermal insulation protecting said valve from external fire.

14. A shipping cask for spent nuclear fuel assemblies, comprising:

means defining a generally cylindrical cask having an open top and a closed bottom;

a closure sealingly closing said open top;

a selected portion of the outer surface of said cask having a body of balsa wood overlying the same to protect said cask against external impacts, said balsa wood being arranged with its grain extending generally perpendicular to said surface, said balsa wood being enclosed in a metal shell secured to said cask over said surface;

generally radially extending reinforcing plates extending through said metal shell and said balsa wood and being secured to said cask; and

a layer of asbestos between the outer surface of said balsa wood and said shell,

15. A shipping cask as defined in claim 14 wherein said surface is the bottom of said cask, said sheel extending outwardly of the periphery of said cask and serving as a broad base for supporting said cask in an upright position 16. A shipping cask as defined in claim 14 wherein said shell overlies said closure and open end of said cask and is removably secured to said closure and sealed to said open end.

17. A shipping cask for spent nuclear fuel assemblies. comprising:

means defining a generally cylindrical hollow enclosure having a closed end and an open end;

removable elongated generally flat plates of gamma ray shielding material arranged to extend chordally and loosely positioned in said enclosure and defin partment is the interior of a unitary removable container.

18. A cask as defined in claim 17 including foraminous material extending across the spaces between said plates whereby to retain fuel particles from any ruptured fuel rods in said compartment while permitting circulation of coolant therethrough. 

1. A shipping cask for spent nuclear fuel assemblies, comprising: means defining a generally cylindrical enclosure having a closed end and an open end; gamma ray shielding material surrounding said enclosure; neutron shielding means extending circumferentially around said gamma ray shielding material comprising means defining a plurality of separate longitudinally extending compartments for bodies of liquid material capable of absorbing neutrons; pressure relief means for each of said compartments; and each of said compartments comprising a main portion and a surge tank portion separated therefrom by a partition, a small passageway through said partition, and means at said passageway for holding a body of said liquid in closing relation to said passageway throughout a substantial range of tilting of said cask to thereby prevent air from flowing from said surge tank portion into said main tank portion.
 2. A shipping cask as defined in claim 1 and including: a first pair of radially outwardly extending diametrically opposed and axially aligned trunnions adjacent said open top; and a second pair of opposed and axially aligned trunnions extending outwardly from adjacent said closed bottom and from opposite sides of said cask with their aligned axes parallel to the axes of the trunnions of said first pair, the trunnions of said second pair being laterally offset from the longitudinal axis of said cask whereby when said second pair of trunnions is engaged with a supporting cradle, with said cask upright, said cask will be unbalanced and tend to tilt toward a horizontal position.
 3. A cask as defined in claim 1 wherein each of said compartments is provided with a solder filled opening in its outer wall, serving as a blow-out plug.
 4. A cask as defined in claim 1 wherein said bodies of liquid material comprise potassium tetraborate, water and ethylene glycol.
 5. A cask as defined in claim 4 wherein said ethylene glycol comprises about 70% of said body of liquid material and about 0.6% of said body of liquid material is boron.
 6. A cask as defined in claim 1 wherein said compartments extend axially of said cask less than the full length of said enclosure, having their ends inwardly of the ends of said enclosure.
 7. A shipping cask for spent nuclear fuel assemblies, comprising: means defining a generally cylindrical enclosure having a closed end and an open end; gamma ray shielding material surrounding said enclosure; neutron shielding means extending circumferentially around said gamma ray shielding material comprising means defining a plurality of separate longitudinally extending compartments for bodies of liquid material capable of absorbing neutrons; pressure relief means for each of said compartments; said gamma ray shielding comprising radially spaced generally concentric tubes of stainless steel; an annular body of lead substantially but not completely filling the space between said tubes; a body of stainless steel defining said closed end and being wElded to the adjacent ends of said tubes; an annular flange of stainless steel welded to the other ends of said tubes and defining said open end; and a stainless steel closure secured to said flange and being sealed thereto.
 8. A cask as defined in claim 7 wherein said closure is sealed to said flange by a pair of spaced O-ring gaskets, and a gasket leak check valve carried by said flange and communicating with the space between said O-rings.
 9. A cask as defined in claim 7 including metal vanes secured to at least one of said tubes and extending into said body of lead to enhance heat transfer therebetween.
 10. A shipping cask for spent nuclear fuel assemblies, comprising: means defining a generally cylindrical enclosure having an open top and closed bottom and comprising thick metal members, of gamma ray shielding material, defining said closed bottom and said open top; a radial cavity extending inwardly from the outer surface of said cask into at least one of said members; a passageway extending inwardly from said cavity to a space within said cask; and valve means for closing said passageway, said valve means being entirely within said cavity within said gamma ray shielding material and thereby protected from damage by external impacts.
 11. A cask as defined in claim 10 wherein said valve comprises a rupture disc device adjacent said passageway and a pressure relief valve in series therewith.
 12. A cask as defined in claim 10 including an annular structure around said one member; an opening through said structure and communicating with said cavity; said valve having an outlet port and a frangible tube extending outwardly from said outlet port into said opening; and a normally closed, selectively openable valve at the outer end of said tube adjacent the outer end of said opening.
 13. A cask as defined in claim 12 including a removable closure sealingly closing the outer end of said opening and including thermal insulation protecting said valve from external fire.
 14. A shipping cask for spent nuclear fuel assemblies, comprising: means defining a generally cylindrical cask having an open top and a closed bottom; a closure sealingly closing said open top; a selected portion of the outer surface of said cask having a body of balsa wood overlying the same to protect said cask against external impacts, said balsa wood being arranged with its grain extending generally perpendicular to said surface, said balsa wood being enclosed in a metal shell secured to said cask over said surface; generally radially extending reinforcing plates extending through said metal shell and said balsa wood and being secured to said cask; and a layer of asbestos between the outer surface of said balsa wood and said shell.
 15. A shipping cask as defined in claim 14 wherein said surface is the bottom of said cask, said sheel extending outwardly of the periphery of said cask and serving as a broad base for supporting said cask in an upright position.
 16. A shipping cask as defined in claim 14 wherein said shell overlies said closure and open end of said cask and is removably secured to said closure and sealed to said open end.
 17. A shipping cask for spent nuclear fuel assemblies, comprising: means defining a generally cylindrical hollow enclosure having a closed end and an open end; removable elongated generally flat plates of gamma ray shielding material arranged to extend chordally and loosely positioned in said enclosure and defining at least one generally square compartment therebetween for a fuel assembly; and defining spaces between said plates and the inner periphery of said enclosure, for coolant flow; means providing for flow of coolant in said enclosure into said out of said compartment; adjacent edges of said plates being spaced apart and opposite ends of said plates being secured to end members common to said plates whereby said compartment is the interior of a Unitary removable container.
 18. A cask as defined in claim 17 including foraminous material extending across the spaces between said plates whereby to retain fuel particles from any ruptured fuel rods in said compartment while permitting circulation of coolant therethrough. 